The invention relates to a diffusor for decelerating a fluid, having at least one wall that forms a channel, having an inlet cross section and having an outlet cross section, with the flow cross section of the channel at the outlet cross section being larger than at the inlet cross section.
Such diffusors are found in a large number of conventional continuous flow machines. For example, the flow rate of the steam is reduced in a diffusor at the low-pressure end of a steam turbine, so that:
the usable pressure or enthalpy drop across the turbine is increased;
a proportion of the kinetic energy is converted to pressure energy;
the flowing medium is decelerated; and
the flow losses at the diffusor outlet toward the condenser groups are reduced.
The decreased supply of heat resulting from dissipation also reduces the cooling power required in the condenser that is positioned downstream in steam turbines.
Similarly, a draft tube is connected to Francis turbines through which a liquid flows to achieve the conversion from velocity energy to pressure energy mentioned above, and, thus, to increase the power of the turbine. In the case of gas turbines and other apparatuses through which flow passes at a high velocity, it is also desirable to recover at least a portion of the flow energy of the fluid by fitting a diffusor.
All types of diffusors have one fundamental problem in that separation effects can occur between flow and the wall of the diffusor due to the decelerated flow. These separation effects invariably reduce the free cross section of the diffusor and, hence, at least partially cancel out its effect.
Furthermore, the separation effects can result in what are referred to as shock boundary layer pulsations (hereinafter referred to as pulsations) in the fluid that can cause oscillation of the turbine blades, the casing of an upstream turbine, and the casing of the diffusor. These oscillations are highly undesirable because they represent an additional mechanical stress on the components affected by them, and can, thus, reduce the operational reliability and the life of these components to a major extent.
For such a reason, in the past, attempts have been made in widely differing ways to prevent the occurrence of shock boundary layer pulsations in diffusors, or to reduce their amplitude.
For example, attempts have been made to fit guide vanes in the channel of the diffusor to prevent detachment of the flow from the convex-curved wall of the diffusor. Such a configuration reduces the pulsation. However, the diffusor efficiency is reduced significantly throughout the entire operating period due to the permanently installed guide vanes.
Another approach to reducing pulsations is to increase the ratio of the outlet cross section to the inlet cross-section of the diffusor (pressure gradient variation). Even such a measure has not made it possible to entirely suppress shock boundary layer pulsations.
Furthermore, attempts have been made to use a kink in the wall of the diffusor to deliberately cause flow separation from the wall. The configuration allows the pulsations to be suppressed completely. However, the measure is associated with a considerable reduction in the diffusor efficiency.
It is accordingly an object of the invention to provide a diffusor without any pulsation of the shock boundary layer, and a method for suppressing the shock boundary layer pulsation in diffusers that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that provides a diffusor and a turbine having relatively good efficiency throughout the entire operating envelope and throughout the entire operating life, and during whose operation no shock boundary layer pulsations occur. Furthermore, the invention provides a method for effectively suppressing pulsations in a diffusor.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a diffusor for decelerating a moving fluid including at least one wall forming a channel for receiving a moving fluid, the at least one wall having an inlet, an outlet, and at least one opening for receiving an energizing fluid to be transported selectively into the channel, the inlet having a relatively smaller inlet flow cross section and the outlet having a relatively larger outlet flow cross section.
Passing the energizing fluid into the channel results in the deliberate supply of energy to the fluid whenever pulsations occur to suppress the pulsation and prevent damage to the turbine blades of an upstream turbine or of the diffusor.
In accordance with another feature of the invention, the inlet opening or openings are circular in shape or in the form of an elongated or elliptical hole. Thus, the openings are easy to produce and have only a small notch effect.
In accordance with a further feature of the invention, the inlet opening or openings are disposed in at least one or more regions of the wall, in particular, in those regions in which pulsation of the shock boundary layer between the fluid and the wall occurs so that the extent to which the wall of the diffusor is weakened by the inlet openings remains low. Furthermore, passing of the energizing fluid into the region or regions of the wall in which pulsation of the shock boundary layer occurs deliberately influences or suppresses the pulsation.
In accordance with an added feature of the invention, the channel has an annular cross section so that diffusers having an inner shell and a convex-curved outer shell can also be operated reliably and with high efficiency at all operating points.
In accordance with an additional feature of the invention, the fluid arrives in the diffusor in an axial direction and/or has a swirl in the inlet cross-section and/or the fluid emerges from the diffusor in the radial direction. Thus, a diffusor according to the invention can easily be installed between a steam turbine and a condenser with widely differing inlet and outlet flow conditions.
In accordance with yet another feature of the invention, to simplify production and to improve the flow conditions, the diffusor and/or the wall of the diffusor are rotationally symmetrical.
In accordance with yet a further feature of the invention, there is provided at least one pressure sensor on the diffusor. The pressure sensor measures the pressure of the fluid in a non-stationary manner so that continuous monitoring for the occurrence of pulsations during operation is feasible.
In accordance with yet an added feature of the invention, the pressure sensor measures pressures of the moving fluid at various locations in the channel.
In accordance with yet an additional feature of the invention, there is provided a controller. The controller determines amplitudes and frequencies of the pressures measured by the pressure sensor, controls movement of the energizing fluid into the diffusor, and initiates the movement of the energizing fluid into the diffusor when the amplitudes within a predetermined frequency band exceed a threshold value. The configuration ensures, on one hand, that energizing fluid is passed into the diffusor whenever pulsations occur and, on the other hand, that the movement of the energizing fluid is prevented when no pulsations are measured. The configuration has no adverse effect whatsoever on the efficiency of the diffusor according to the invention at those operating times at which no pulsations occur, and the diffusor efficiency is reduced to only a very minor extent just during the comparatively short operating periods in which a pulsation occurs. The efficiency of the diffusor according to the invention is, therefore, just as good, in all operating situations, as the efficiency of a diffusor according to the prior art when no pulsations are occurring in such a diffusor. Comparatively, however, the efficiency of a diffusor according to the invention when no pulsations occur is considerably better than the efficiency of a diffusor according to the prior art in similar conditions. The operational reliability of a turbine equipped with a diffusor according to the invention is considerably better than that in the prior art.
In accordance with again another feature of the invention, the energizing fluid has the same consistency, or a similar consistency, or a different consistency, to that of the fluid. Thus, an energizing fluid is available, cost-effectively and without any additional hardware, for example, by tapping off part of the steam flow in the medium-pressure or low-pressure part of the upstream steam turbine, whose parameters (pressure, temperature, mass flow) can be set precisely to the operational purpose. For example, steam from a tapping line from a steam turbine can be used as energizing fluid.
In accordance with again a further feature of the invention, the energizing fluid is compressed air so that the pulsations can be suppressed without any changes to the turbine or to any other apparatus upstream of the diffusor according to the invention.
With the objects of the invention in view, there is also provided a turbine including a rotor, a casing for receiving a flow of fluid to drive the rotor, the rotor being disposed in the casing and being driven by the fluid, and a diffusor for decelerating the fluid. The diffusor is disposed in the casing downstream of the rotor in a flow direction of the fluid and has at least one wall forming a channel for receiving the fluid. The at least one wall has an inlet, an outlet, and at least one opening for receiving an energizing fluid to be transported selectively into the channel. The inlet has a relatively smaller inlet flow cross section and the outlet has a relatively larger outlet flow cross section. Thus, all of the advantages mentioned above related to the diffusor according to the invention are also present in a turbine according to the invention.
In accordance with again added features of the invention, the turbine is a steam turbine, a low-pressure steam turbine, a gas turbine, a water turbine, or a Francis turbine so that pulsation in the diffusor of the steam or gas turbine or in the draft tube of the Francis turbine is suppressed regardless of the various fluids with which the turbines are operated.
With the objects of the invention in view, there is also provided a method for preventing shock boundary layer pulsations in the diffusor, in which:
the pressure of the fluid in the diffusor is measured in a non-stationary manner;
the amplitudes and frequencies of the measured pressures are evaluated; and
energizing fluid is passed into the diffusor if the amplitudes within a predetermined frequency band exceed a threshold value.
With the objects of the invention in view, there is also provided a method for preventing shock boundary layer pulsations in a diffusor including the steps of measuring pressures of a moving fluid at various locations in a diffusor, evaluating amplitudes and frequencies of the measured pressures, and feeding energizing fluid into the diffusor if the amplitudes within a predetermined frequency band exceed a threshold value.
The methods according to the invention makes it possible, whenever an operating point is approached at which pulsations occur, for these pulsations to be measured, identified, and suppressed by passing energizing fluid into the diffusor. Thus, inter alia, even existing diffusers in which pulsations occur can be monitored, and the pulsations can be suppressed by passing energizing fluid into the diffusor. Accordingly, all relatively old power stations can be retrofitted with the method according to the invention to allow a diffusor optimized according to the present-day prior art to be installed in all power stations without any risk of the stations being subjected to oscillations. Relatively modern power station complexes, including those presently being constructed, can, of course, also be retrofitted with the method according to the invention.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a diffusor without any pulsation of the shock boundary layer, and a method for suppressing the shock boundary layer pulsation in diffusers, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.